Fading, as the term is used in the Computer Image Generation (CIG) arts, refers to the modification of the intrinsic feature color chromicity and intensity of an object towards a common haze or fading color, as a function of both the range between the object and observer and the density of the scatterers (e.g. fog droplets, dust particles and the like). The physical basis of this phenomena is illustrated, for a relatively simple case, in FIG. 1; the viewer 10, located at some viewpoint VP, is attempting to view a desired point 11p at the location P on a surface 11a from which illuminating light is reflected from an object 11. Light rays 12a emanate from a source 12, i.e. the sun, and then perhaps pass through a haze layer 14 (e.g. a layer of suspended moisture droplets) on the way to the object 11 being viewed. A portion 14s1 of the light 12a incident on, and passing through, layer 14 is scattered by the layer and the light rays 12b actually illuminating object 11 are thus modified by the haze to have different chromicity and attenuated intensity. With the moisture particles acting as miniature prisms, the scattered light 14s1 is refracted to create a multi-color effect the sum of which typically appears near-white. If the viewer 10 is so located as to cause the light rays 12c reflected from the viewed object 11 to pass through the layer 14 (or some other scattering layer), some of rays 12c are themselves scattered, as scattered light 14s2, so that the light rays 12d actually reaching the viewpoint are even further modified in amplitude; the attenuation may, dependent upon the nature of the scattering layer or layers, be modified in chromicity as well as amplitude. Some of the scattered light 14s1/14s2 may be returned in the direction of viewer 10, thereby creating an additional color source for the eye.
Since a portion of the light from the source is attenuated by the time it is received by the viewer, the direct contribution of the diffuse illumination of the object 11 will be diminished. Mimicking this effect is a critical capability for CIG systems (such as that using the Advanced Video Object Generator disclosed and claimed in U.S. Pat. No. 4,727,365, incorporated herein in its entirety by reference) and especially so when the CIG is used as part of a training system (e.g. a flight simulator and the like). Most critical is the ability to provide a realistic model of the effects of a multilayer atmospheric model, with the effects of each layer calculated at the pixel level. This capability will enable a system to be capable of training in true ground fog situations in which individual objects are accurately modeled, even though the objects exist partially in the haze and partly in a clearer layer. This capability is of particular importance in rotor-wing aircraft training and armor applications, as the majority of such training is at or near ground level, where fog greatly impacts the visibility of the environment. Accordingly, it is highly desirable to provide a multi-layer scattering capability to real-time CIG systems.